A wide variety of iron-sulfur clusters and related mixed-metal species are found as active sites in metalloenzymes. In some cases these proteins form part of electron transport chains, and in other cases they serve as catalytic centers for quite unusual chemistry. Our research focusses on the use of modern techniques of quantum chemistry to carry out theoretical studies (at the spin Hamiltonian and density function level) of their electronic structures, in an attempt to more closely connect the spectroscopy of these systems to their structure and function. A new emphasis places increased attention on the spin Hamiltonians used to interpret experimental data, and to ways in which these can be related to detailed computer calculations. The next project period will emphasize the FeS and MoFeS clusters in nitrogenase, the FeS cubane-siroheme system in nitrite and sulfite reductase, and the substrate-bound forms of aconitase. Specific projects include; (a) analysis of ligand hyperfine interactions and paramagnetic NMR shifts in 2Fe and 4Fe cluster; (b) use of our newly- developed model of environmental effects to calculate redox potentials of iron-sulfur clusters in model complexes and in proteins, including the effects of the protein (and amino acid substitution) on redox potential; (c) development of density functional and spin-coupling models for the P the active site of sulfite reductase in its resting and hypothetical intermediate forms; (e) detailed analysis of spin coupling and hyperfine interactions in the active sites of Rieske proteins; (f) further development of computer programs to compute fundamental spectroscopic properties from density functional theory, and to prepare useful graphical representations of charge and spin densities and their associated electrostatic potentials.